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Old 16th June 2012, 04:47 AM   #1
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Default Another Adcom Amp Question

I've recapped a couple of GFA-555 amps with Panasonic FC series and I'm getting ready to do a pair of GFA-545 II amps.

In all of these, there is a 4.7uF cap in parallel with each bias trimmer pot. I'm thinking of replacing these with Wima MKS 2 Metallized Polyester (PET)
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Any thoughts from the experts as to whether this is a good idea from the standpoint of reliability and sound? It seems like a good idea to replace the electrolytics with something more long-lived, but I don't know if there are other characteristics that might make this swap less than completely successful.
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Old 16th June 2012, 08:06 AM   #2
djk is offline djk
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The point of the cap is to prevent the signal from modulating the bias.

I would use a larger value, you may want to listen to it before putting it back together for good.

I have seen Nelson Pass use 4.7F, 10F, and 220F in that location in some of his designs.
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Old 16th June 2012, 01:24 PM   #3
fpitas is offline fpitas  United States
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Quote:
Originally Posted by djk View Post
The point of the cap is to prevent the signal from modulating the bias.

I would use a larger value, you may want to listen to it before putting it back together for good.

I have seen Nelson Pass use 4.7F, 10F, and 220F in that location in some of his designs.
I agree with the bigger cap principle here. Rod Elliott, who is usually a reasonable fellow, also likes big caps across the bias circuit, and claims it makes a difference:

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About halfway down the page, under Description.

Last edited by fpitas; 16th June 2012 at 01:26 PM.
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Old 17th June 2012, 06:38 PM   #4
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Quote:
Originally Posted by djk View Post
I have seen Nelson Pass use 4.7F, 10F, and 220F in that location in some of his designs.
Thanks for the replies.

I'm hesitant to second-guess Nelson Pass when it comes to component values (I view using improved quality parts of the same rating as being a more conservative approach I am comfortable with).

Since he used different values of caps (as well as different values of trimmer pots) in different amps, I have to wonder why. I can't believe cost was the driving factor, given that small value el caps are pennies each to a manufacturer. I also doubt that the choice was made randomly.

P.S. I'm not one of the listen and judge types. I know too much about human psychology to believe that I, alone in the human race, am immune to expectation bias.
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